1. Field of Invention
This invention relates to encapsulation of SAW devices, and more particularly to a method and apparatus for vacuum encapsulation of a SAW device.
2. Description of the Prior Art
SAW devices are well known in the art and find utility in a number of different applications including high frequency oscillators, pressure transducers, and encoders/correlators of PSK encoded signal information. The outstanding features of the SAW devices are: their ability to provide real time delays of electromagnetic waves within comparatively short, acoustic wavelength substrate materials, and their inherent rugged construction, permitting their use in high vibration and G force environments. The use of SAW delay lines in high frequency oscillator circuits is well known, and is particularly attractive as an alternative to bulk wave resonators which are highly sensitive to vibration, however, the high accuracy and stable frequency characteristics of the bulk resonators require that any competing SAW oscillator be protected from contamination which changes the acoustic wave propagation velocity through the SAW substrate. Any change in velocity results in a change in the SAW time constant (the real time delay provided by the SAW) which in turn causes frequency shifting of the SAW oscillator. Typical of the high accuracy requirements for the bulk wave resonators is the aging requirement, i.e. the result of change in the bulk resonator fundamental frequency with time, which as reported in a paper entitled "A New Ceramic Flat Pack For Quartz Resonators", by T. D. Wilcox et al. presented at the 29th Annual Frequency Control Symposium (1975), is on the order of two parts in 10.sup.10 per week, for a 5 megahertz fundamental mode crystal unit. The severity of this aging requirement may better be appreciated by the statement by Wilcox et al that a change of one monolayer (single atomic layer) on the surface of a quartz crystal (molecules landing on, or driven off of the surface) corresponds to a frequency change of about one part in 10.sup.6 (0.0001%). Since the surface acoustic wave propagates across the SAW substrate with atomic particle motion confined to a depth of approximately one wavelength from the surface of the substrate, the problem of surface contamination of the SAW substrate is particularly acute. Therefore, any practical utilization of SAW oscillators as replacements for the bulk resonators requires that the SAW device be vacuum encapsulated.
Vacuum encapsulation of SAW devices, however, presents further problems affecting stability which may be more severe than surface contamination, specifically the thermal strain induced in the SAW substrate from thermal expansion and contraction of the encapsulating structure. These induced thermal strains provide distortion of the SAW substrate resulting in changes in the wave propagation velocity and, therefore, the SAW time constant. The prior art packaging techniques used in related electronic industries, such as the semi-conductor industry, and the packaging techniques for bulk resonators, are not appropriate for providing vacuum encapsulation of the SAW devices. As an example, the encapsulation of bulk crystal resonators, such as the state of the art package reported by Wilcox et al, provides for thermal and mechanical isolation of the resonator from the encapsulating structure by suspension of the resonator through its electrical interconnecting leads within the package, thereby isolating the resonator from the package thermal expansion characteristics, and allowing free movement of the resonator. This, however, results in the resonators susceptability to vibration. While a similar arrangement for a SAW device may be provided, i.e. suspension of the SAW substrate in the package through the interconnections to its electrodes, this would effectively eliminate one of the main attractions of the SAW device, its ability to withstand high vibration environments. Similarly, the semi-conductor industry technique of providing metal can encapsulation of the integrated circuits is unsuitable due to the dissimilar thermal characteristics of the metal can, which if bonded to the SAW substrate produces undesirable strain in the substrate causing changes in the acoustic wave velocity and SAW time constant.